This invention relates to an apparatus and method for use in the quantitative analysis of a blood sample or similar type of fluid suspension.
Blood is a fluid suspension made of three basic components, plasma, red blood cells, and a buffy coat component (which includes white blood cells and platelets). The red blood cells are the most dense of the three components, and the plasma is the least dense. Further, the red blood cells and plasma comprise relatively large portions of the blood, and the buffy coat component comprises a relatively small portion of the blood.
An old and well known technique for making a quantitative analysis of a person's blood involves centrifuging a sample of blood with a device called a Wintrobe tube. Basically, a test tube is filled with a blood sample, and spun about a vertical axis. The test tube is oriented so that it extends away from the vertical axis, and as the tube is spun, centrifugal forces cause the components of the blood to seperate into layers. The tube diameter is basically constant over its length, and the length of the component layers is arithmetically proportional to their relative volume percentage of the blood sample. Thus, by measuring the length of a component layer, the relative amount of that component in the sample can be determined.
The foregoing technique has been a reasonably good one for determining the amount of red blood cells in a blood sample. However, it has not been a good technique for quantifying the white cells, because the buffy coat component occupies a very small portion of the sample, and becomes sandwiched into a narrow band between the larger volumes of red blood cells and plasma. Measurement of that narrow band is a difficult task.
Recently, a technique has been suggested for physically enlarging the axial extent of the buffy coat component in order to measure it. Specifically, U.S. Pat. No. 4,027,660 suggests insertion of a special loose plug into the centrifugal tube. The plug has a specific gravity such that it will float upon, or slightly in, the red blood cell portion. Thus, it is designed to axially spread the buffy coat component so it can be more readily measured. U.S. Pat. Nos. 4,077,396; 4,082,085; and 4,091,659 show some variations on that concept.
Other techniques for use in the analysis of a blood sample are shown in U.S. Pat. Nos. 2,822,126; 3,291,693; 3,415,627; 3,679,367; 3,684,450; 3,713,775; 3,880,592; and 4,035,156. Additionally, other types of devices for use in separating or centrifuging blood or other fluid suspensions are shown in U.S. Pat. Nos. 3,291,387; 3,556,302; 3,748,101; and 4,007,871.